1. Field of the Invention
The present invention relates to an optical switch, and more particularly relates to an optical switch of a type which is improved by providing a light output intensity variable attenuation function to a two input two output crossbar type optical switch which is widely used in optical add/drop modules for wavelength multiplex optical communication and the like.
2. Description of the Related Art
In an Optical Add/Drop module (hereinafter abbreviated as an “OADM”) for high Dense Wavelength Division Multiplexing (hereinafter abbreviated as “DWDM”) optical communication systems which has become widespread in recent years, a plurality of two input two output crossbar type optical switches are commonly used in accordance with the number of wavelength channels. FIG. 13 shows an example of the structure of the OADM for such DWDM communication systems. In FIG. 13, the reference symbol 1 denotes a wavelength demultiplexer (hereinafter abbreviated as DEMUX), while the reference symbol 2 denotes a wavelength multiplexer (MUX); and the OADM is constituted by the structure from this DEMUX 1 up to the MUX 2. And the reference symbol 3 denotes add ports while the reference symbol 4 denotes drop ports, and these ports are connected via optical switches 5.
In this OADM, the input light which has been wavelength division multiplexed is divided into light of various wavelengths by the DEMUX 1, and is inputted via optical path 7 to one of the input ports of each of the optical switches 5, while light which has been inputted from one of the add ports 3 is inputted to the other input port of each of the optical switches 5. The light which is inputted to these optical switches 5 is dispatched, by switching over their optical paths according to requirements, either to their drop ports 4 or to the MUX 2, and the light which is thus dispatched to this MUX 2 is wavelength division multiplexed and outputted.
The reference symbol 6 denotes Variable Optical Attenuatiors (hereinafter abbreviated as “VOA”s), one of which is inserted into each of the channels, with the objective of equalizing the light intensity between the channels. Accordingly, in the prior art type OADM shown in FIG. 13, a VOA 6 is connected to each output port for outputting from the optical switch 5 to the MUX 2. Many types of two input two output crossbar type optical switches and VOAs are on the market as individual optical components, and various ones thereof are also in use in OADMs.
FIG. 1 shows, the polymer thermo-optic digital optical switch, as one example of such an optical switch 5 of a two input two output crossbar type optical switch. This optical switch consists of an embedded polymer optical waveguide 11 on a silicon wafer and metal thin films of the surface of the waveguide 11. One example of the polymer waveguide material is polyimide. The mater thin films are made of nickel, aluminum, chrome, gold, nichrome or the and formed by a vapor deposition method, a spattering method, or the like. The electrode pads are also formed upon the surface of the optical waveguide 11. The core pattern of these optical waveguide 11 is that four Y branches are mutually opposed.
The metal thin films are arranged so as to be positioned above the respective Y branches at left and right sides thereof. Electrode pads for connection wiring are provided at both ends of each of these metal thin films, and are connected to electrode pins of a package case by gold wire bonding, so as to be connected to an external power source. When electric current comes to be flowed through these metal thin films by an external power source, they function as heaters due to the Joule heat which is generated in them at this time.
The refractive index of the polymer materials such as polyimide or the like which is used as the optical waveguide 11 decreases when the temperature of the materials increases. And light is propagated through the high refractive index area of this medium. Due to this, by heating up only one of the metal thin films which are provided upon the left and right sides of the Y branch, it is possible to direct the light to the side of the optical waveguide 11 that is not heated up.
In these metal thin film which thus function as heaters, the reference symbols 12a denote outer side heaters which are provided at the outer sides of the optical waveguide 11, while the reference symbols 12b denote inner side heaters which are provided at the inner sides of the optical waveguide 11. Accordingly, four of these outer side heaters 12a and four of the inner side heaters 12b are provided to the two input two output optical switch shown in FIG. 14.
FIG. 15 shows an example of a control method for optical path switching over of this two input two output crossbar optical switch. FIG. 15 shows the system in the cross state, in which, among the metal thin film heaters which are formed upon the waveguide, the four outer side heaters 12a are supplied with electrical power from a DC source 21 and are heated up. At this time, the light which is inputted from the input port 1 is outputted at the output port 2, while the light which is inputted from the input port 2 is outputted at the output port 1. On the other hand, the system can be switched over to the bar state by heating up the four inner side heaters 12b. At this time, the light which is inputted from the input port 1 is outputted at the output port 1, while the light which is inputted from the input port 2 is outputted at the output port 2.
FIG. 16 shows the way in which the connection wiring, after being brought together in common within the optical switch component, is connected to the DC source. By doing this, it is possible to provide the same function as the above described control method, while simplifying the external wiring. FIG. 16 shows the cross state, and, in the optical switch 5 in this figure as well, the reference symbols 12a denote the outer side heaters and the reference symbols 12b denote the inner side heaters, while the reference symbol 21 denotes the DC source.
Since one VOA is fitted to and used for each channel when this type of optical switch is used in an OADM, it has been attempted to integrate VOAs into such a two input two output crossbar type optical switch. As such a two input two output crossbar type optical switch integrated with VOAs, there may be cited as an example the one disclosed in Japanese Patent Application, First Publication No. 2002-196378 and due to the present inventors. In this optical switch, thermo-optic VOAs are integrated with a two input two output crossbar type optical switch which consists of four one input two output polymer thermo-optic digital optical switches opposed to one another.
However, with the optical switch disclosed in Japanese Patent Application, First Publication No. 2002-196378, a heater is provided for the VOAs, in addition to the eight heaters for the two input two output type crossbar optical switch, and due to this it is necessary to make the element larger, as compared to only a two input two output type crossbar optical switch. Furthermore, as shown in FIG. 14, in order to add a light output intensity variable attenuation function to the above described optical switch, it is also possible to extend the optical waveguide on the output port side, and to provide one additional heater 13. However, the element length of the two input two output optical switch becomes longer to this extent, and the element area also becomes greater.
Just as with a semiconductor product like a CPU or a DRAM, the cost of an optical switching element is greatly influenced by how many elements can be made from a single silicon wafer. Accordingly, the two input two output optical switch of this type which is endowed with a light output intensity variable attenuation function and which incorporates nine heaters has a higher cost in proportion to its increase in length. Furthermore, in a macromolecular optical waveguide in which a certain amount of propagation loss is caused by material characteristics, add to increase of element cost and of module size entailed as described above, the problem arises that it is not possible to ignore the increase in insertion loss due to increase in the length of the optical path.
In the case of a Silica Planer Lightwave Circuit (hereinafter abbreviated as PLC) devices, for example, the propagation loss of embedded waveguide is about 0.02 dB/cm, so that increase of this order of the length of the optical path will not present a problem. However, in case of usual polymer PLC devices, the propagation loss is larger than 0.1 dB/cm. For example, the propagation loss of fluorinated polyimide PLC devices is about 0.6 dB/cm at 1550 nm band. So that the increase of the propagation loss causes by a few millimeters increase of the length of the element is not negligible for polymer PLC devices.